Wireless communication systems are widely deployed to provide various types of communication content such as voice, data, and so on. These systems may be multiple-access systems capable of supporting communication with multiple users by sharing the available system resources (e.g., bandwidth and transmit power). Examples of such multiple-access systems include code division multiple access (CDMA) systems, time division multiple access (TDMA) systems, frequency division multiple access (FDMA) systems, 3GPP Long Term Evolution (LTE) systems, and orthogonal frequency division multiple access (OFDMA) systems.
Generally, a wireless multiple-access communication system can simultaneously support communication for multiple wireless terminals. Each terminal communicates with one or more base stations via transmissions on the forward and reverse links. The forward link (or downlink) refers to the communication link from the base stations to the terminals, and the reverse link (or uplink) refers to the communication link from the terminals to the base stations. This communication link may be established via a single-in-single-out, multiple-in-single-out or a multiple-in-multiple-out (MIMO) system.
A MIMO system employs multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NS independent channels, which are also referred to as spatial channels, where NS≦min{NT, NR}. Each of the NS independent channels corresponds to a dimension. The MIMO system can provide improved performance (e.g., higher throughput and/or greater reliability) if the additional dimensionalities created by the multiple transmit and receive antennas are utilized.
A MIMO system may support time division duplex (TDD) and/or frequency division duplex (FDD) systems. In a TDD system, the forward and reverse link transmissions are on the same frequency region so that the reciprocity principle allows the estimation of the forward link channel from the reverse link channel. This enables the base station to extract transmit beamforming gain on the forward link when multiple antennas are available at the base station. In an FDD system, forward and reverse link transmissions are on different frequency regions.
In a FDD transceiver, the strongest interference on a received signal may be caused by self-jamming leakage from a transmit (Tx) signal that is simultaneously transmitted by the transceiver. The Tx signal may leak to the receive path through the finite isolation (e.g., through a duplexer filter, antenna coupling, circuit card electromagnetic interference (EMI), and VLSI chip coupling). Although in a different frequency band, the Tx leakage signal may cause co-channel interference on the intended received signal due to excitations of some non-linear behavior in the Rx. This scenario is referred hereinafter as self-jamming. The co-channel interference may be generated when nonlinearities are excited in the radio frequency (RF) downconversion components: such as low noise amplifier (LNA), mixer, switches, filters, data converters and other like components.
Two of the most relevant effects of nonlinearities are second-order inter-modulation distortion (IM2), and third order cross-modulation (Xmod). The Xmod is a combination of the Tx leakage plus adjacent narrow-band interference. As will be described in greater detail below, higher order interference non-linearities (both even-order and odd-order) may also be cancelled utilizing a similar scheme, all of which fall in the scope of the disclosure.
The problem of transmit signal leakage and subsequent potential IM2 and Xmod distortion in a frequency band may be addressed in different ways. A predominant type of solution utilizes analog RF approach which detects presence of an adjacent jammer and increases the current drawn in the mixer to improve linearity. Obviously, this approach results in higher power consumption and hence reduces talk-time of a wireless device. In certain scenarios existing analog solutions may not be enough to keep non-linear distortion signals from negatively impacting the receiver performance. In both of these example scenarios, an improved solution is desirable.